FROMLIST: x86/boot: Remove run-time relocations from head_{32,64}.S

The BFD linker generates run-time relocations for z_input_len and
z_output_len, even though they are absolute symbols.

This is fixed for binutils-2.35 [1]. Work around this for earlier
versions by defining two variables input_len and output_len in addition
to the symbols, and use them via position-independent references.

This eliminates the last two run-time relocations in the head code and
allows us to drop the -z noreloc-overflow flag to the linker.

Move the -pie and --no-dynamic-linker LDFLAGS to LDFLAGS_vmlinux instead
of KBUILD_LDFLAGS. There shouldn't be anything else getting linked, but
this is the more logical location for these flags, and modversions might
call the linker if an EXPORT_SYMBOL is left over accidentally in one of
the decompressors.

[1] https://sourceware.org/bugzilla/show_bug.cgi?id=25754



Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Fangrui Song <maskray@google.com>
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/lkml/20200629140928.858507-7-nivedita@alum.mit.edu


Bug: 167402429
Signed-off-by: Alistair Delva <adelva@google.com>
Change-Id: Ic40c42463ec7c1d15726b39e949743bf62016b42